High-voltage capacitors are useful in technology applications where brief high-voltage pulses must be delivered, for example, in automatic implantable cardioverter/ defibrillators ("ICDs") where high-voltage pulses are required across the defibrillation lead system to effect defibrillation or cardioversion. An ICD and package therefor, including rolled (or "wound") capacitors, are described in U.S. Pat. No. 4,254,775 to Langer.
Implantable defibrillators are implanted in patients suffering from potentially lethal cardiac arrhythmias. The device monitors cardiac activity and decides whether electrical therapy is required. If a tachycardia is detected, pacing or cardioversion therapy may be used to terminate the arrhythmia. If fibrillation is detected, defibrillation is the only effective therapy.
Both cardioversion and defibrillation require that a high voltage shock be delivered to the heart. Since it is impractical to maintain high voltage continuously ready for use, implantable defibrillators charge energy storage capacitors after detection of an arrhythmia and prior to delivering a shock to the heart.
In implantable defibrillators, as in other applications where space is a critical design element, it is desirable to use capacitors with the greatest possible capacitance per unit volume. One way to increase capacitance per unit area in a flat capacitor is to etch the surface of the anode foil perpendicular to the surface thereof. An implantable cardiac defibrillator with improved flat capacitors is described in U.S. Pat. No. 5,131,388 to Pless et al., which incorporated herein by reference.
Background art describing details of construction of traditional high voltage capacitors used in automatic defibrillators are described by P. J. Troup, "Implantable Cardioverters and Defibrillators," at pp. 704-713 (Current Problems in Cardiology, Vol. XIV, No. 12, December 1989, Year Book Medical Publishers, Chicago), which pages are incorporated herein by reference.
Typically, electrolytic capacitors are used in these applications because they have the most ideal properties in terms of size and ability to withstand relatively high voltage. Aluminum electrolytic capacitors are generally used, having aluminum foil plates rolled into a very small volume. By etching the surface of the aluminum foil, the surface area can be further increased and the capacitance increases accordingly.
After the foil is etched, voltage is applied to the foil through an electrolyte such as boric acid and water or other solutions familiar to those skilled in the state of the art, resulting in the formation of aluminum oxide (Al.sub.2 O.sub.3) on the surface of the anode foil. The thickness of aluminum oxide deposited or "formed" on the anode foil is proportional to the applied voltage, roughly 10 to 15 Angstroms per applied volt. The aluminum oxide layer formed on the foil causes the foil to become brittle. In addition, the brittleness of the foil and its capacitance are both proportional to the depth of the etching and the density of the etch pits, i.e., the number per unit area. Accordingly, the capacitance and thereby the energy density are limited by the brittleness of the formed foil.
Another difficulty with using a highly etched anode foil is experienced when the foil is used in a multi-layer flat capacitor such as the one described in Pless et al. If the anode foils are to be electrically connected together by welding, difficulty is experienced because there is insufficient aluminum present for the weld and, if the forming step is performed prior to assembly of the stack and joining of the anode layers, too much aluminum oxide is present for a reliable weld.
The maximum rated voltage of available single electrolytic capacitors has been in the range of 450 V, in which approximately 3 joules can be stored per cubic centimeter of capacitor volume. As noted by Dr. P. J. Troup, (above), this makes the capacitor "probably the single largest limitation to further miniaturization of implantable defibrillators" given current energy delivery requirements.
The maximum high voltage and energy needed for an implantable defibrillator is in the range of 750 V and 40 joules. This necessitates the use of two capacitors coupled in series. Since each capacitor must store approximately 20 joules, their size is relatively large, and it is difficult to package them in a small implantable device. Currently available implantable defibrillators are relatively large (over 10 cubic inches), generally rectangular devices about an inch thick. The patient who has a device implanted may be bothered by the presence of the large object in the abdomen. For patient comfort, it is desirable to minimize the size of the defibrillators. As noted by Troup, the size of the capacitors has been a critical factor contributing to the size of the defibrillator. A further advantage of reducing the size of the defibrillator is that a smaller device can be implanted in the pectoral region of the patient and the defibrillator case may be used as a defibrillation electrode.
It is therefore an object of the present invention to provide a capacitor that can store a higher amount of energy per unit volume.
It is another object of the invention to provide a method of making a capacitor foil having a portion of its surface which is etched to a high degree.
It is still another object of the invention to provide an anode foil for a stacked capacitor which can be easily welded to other anode foils in the stack.